The application of synthetic polymer chemistry to the field of sports equipment has revolutionized the performance of athletes in many sports. One sport in which this is particularly true is golf, especially as relates to advances in golf ball performance and ease of manufacture. For instance, the earliest golf balls consisted of a leather cover filled with wet feathers. These “feathery” golf balls were subsequently replaced with a single piece golf ball made from “gutta percha,” a naturally occurring rubber-like material. In the early 1900's, the wound rubber ball was introduced, consisting of a solid rubber core around which rubber thread was tightly wound with a gutta percha cover. More modern golf balls can be classified as one-piece, two-piece, and three-piece. One-piece balls are molded from a homogeneous mass of material upon which is molded a dimple pattern. One-piece balls are inexpensive and very durable, but do not provide great distance because of relatively high spin and low velocity. Two-piece balls are made by molding a cover around a solid rubber core. These are the most popular types of balls in use today. In attempts to further modify the ball performance, especially in terms of the distance such balls travel, and the feel transmitted to the golfer through the club on striking the ball, the basic two piece ball construction has been further modified by the introduction of additional layers between the core and outer cover layer. If one additional layer is introduced between the core and outer cover layer, a so called “three-piece ball” results, and similarly, if two additional layers are introduced between the core and outer cover layer, a so called “four-piece ball” results, and so on.
Balata had been used as the primary material for covers of golf balls until the 1960's when SURLYN®, an ionomeric resin made by E.I. DuPont de Nemours & Co., was introduced to the golf industry. Ionomers typically costs less than balata and have better cut or shear resistance. At the present time, ionomers are used as the primary polymer source for either or both of the cover stock and intermediate layers for most two-piece and some three-piece golf balls. The problem with ionomer-covered golf balls, however, is that they often lack the “click” and “feel” which golfers had become accustomed to with balata. “Click” is the sound made when the ball is hit by a golf club while “feel” is the overall sensation imparted to the golfer when the ball is hit.
However, unlike ionomer-covered golf balls, polyurethane- or polyurea-covered golf balls can be made to have the “click” and “feel” of balata and the cut or shear resistance of ionomer. Polyurethanes or polyureas are typically prepared by the reaction of a diisocyanate with a polyol (in the case of polyurethanes) or with a polyamine (in the case of a polyurea). Thermoplastic polyurethanes or polyureas may consist solely of this initial mixture or may be further combined with a chain extender to vary properties such as hardness of the thermoplastic. Thermoset polyurethanes or polyureas typically are formed by the reaction of a diisocyanate and a polyol or polyamine respectively, and an additional crosslinking agent to crosslink or cure the material to result in a thermoset.
In view of the aforementioned advantages of polyurethane and polyurea as a golf ball component, numerous patents have disclosed various formulations for these materials. For example, Hewitt et al., U.S. Pat. No. 4,248,432 discloses a thermoplastic polyester urethane golf ball cover formed from a reaction product of a polyester glycol with para-phenylene diisocyanate (PPDI) or cyclohexane diisocyanate, in the substantial absence of curing or crosslinking agents. U.S. Pat. No. 4,123,061 teaches that a golf ball can be made from a polyurethane prepolymer of polyether and a curing agent, such as a trifunctional polyol, a tetrafunctional polyol or a diamine. U.S. Pat. No. 3,989,568 teaches a three-component system employing either one or two polyurethane prepolymers and one or two curing agents. Both polyol and diamine curing agents are taught by the '568 patent. Dusbiber, U.S. Pat. No. 4,123,061, discloses a polyurethane golf ball cover prepared from the reaction product of a polyether, a diisocyanate and a curing agent. Holloway, U.S. Pat. No. 4,349,657 discloses a method for preparing polyester urethanes with PPDI by reacting a polyester (e.g. prepared from aliphatic glycols having 2-8 carbons reacted with aliphatic dicarboxylic acids having 4-10 carbons) with a molar excess of PPDI to obtain an isocyanate-terminated polyester urethane (in liquid form and stable at reaction temperatures), and then reacting the polyester urethane with additional polyester. Wu, U.S. Pat. No. 5,334,673 discloses a polyurethane prepolymer cured with a slow-reacting curing agent selected from slow-reacting polyamine curing agents and difunctional glycols). Wu, U.S. Pat. No. 5,484,870 discloses golf balls having covers composed of a polyurea composition. The polyurea composition disclosed is a reaction product of an organic isocyanate having at least two functional groups and an organic amine having at least two functional groups. One of the organic isocyanates disclosed is PPDI.
Initially, thermoset polyurethanes or polyureas were prepared in what is known as a one-shot process, in which the three reactants, diisocyanate, polyol or polyamine and the curing agent were combined in one step. However in view of the handling problems associated with the diisocyanate starting materials, more recently a two step process is now ubiquitous in the industry in which the golf ball manufacturer would purchase a so-called prepolymer formed by the reaction of the diisocyanate and the polyol (in the case of polyurethane) or the diisocyanate and a polyamine (in the case of a polyurea). This prepolymer poses fewer toxicity problems than the free isocyanate and is thus easier to handle. To the prepolymer is then added either the chain extender or the curing agent prior to or during golf ball layer formation. This procedure is known as the prepolymer process.
Conventionally, golf ball cover and intermediate layers are positioned over a core or other internal layers using one of three methods; injection molding, casting, or compression molding, depending on whether the polymer component is a thermoplastic or a thermoset. Because of their excellent flowability, thermoplastics, including thermoplastic polyurethanes, can be readily formed around a golf ball core using injection molding. Injection molding of thermoplastics generally involves using a mold having one or more sets of two hemispherical mold sections that mate to form a spherical cavity during the molding process. The pairs of mold sections are configured to define a spherical cavity in their interior when mated. When used to mold an outer cover layer for a golf ball, the mold sections can be configured so that the inner surfaces that mate to form the spherical cavity include protrusions configured to form dimples on the outer surface of the molded cover layer. The mold sections are connected to openings, or gates, evenly distributed near or around the parting line, or point of intersection, of the mold sections through which the material to be molded flows into the cavity. The gates are connected to a runner and a sprue that serve to channel the molding material through the gates. When used to mold a layer onto an existing structure, such as a ball core, the mold includes a number of support pins disposed throughout the mold sections. The support pins are configured to be retractable, moving into and out of the cavity perpendicular to the spherical cavity surface. The support pins maintain the position of the core while the molten material flows through the gates into the cavity between the core and the mold sections. The mold itself may be a cold mold or a heated mold. In the case of a heated mold, thermal energy is applied to the material in the mold so that a chemical reaction may take place in the material.
Unfortunately, golf ball covers comprising thermoplastic polyurethane typically exhibit poor shear-cut resistance. Thus, while thermoplastic polyurethane covers are less expensive to make due to their superior processability, they are not favored due to the resulting inferior ball performance.
In contrast, golf ball layers prepared from thermosets, such as thermoset polyurethanes or polyureas, typically exhibit excellent shear-cut resistance and are much more scuff- and cut-resistant than thermoplastic polyurethanes or polyureas. Because such thermoset polyurethanes or polyureas typically have more desirable mechanical properties than the thermoplastic analogs, it would be beneficial to producers of golf balls to be able to use injection molding to prepare thermoset polyurethane or polyurea covers. Unfortunately, thermoset materials generally are not well suited for injection molding, because as the reactants for thermoset polyurethanes or polyureas are mixed, they begin to cure and become highly viscous while traveling through the sprue and into the runners of the injection mold, leading to injection difficulties.
Thus, thermoset materials typically are formed into a ball layer using a casting process free of any injection molding steps. In a casting process, the thermoset material is added directly to the mold sections immediately after it is created. Then, the material is allowed to partially cure to a gelatinous state, so that it will support the weight of a core. Once cured to this state, the core is positioned in one of the mold sections, and the two mold sections are then mated. The material then cures to completion, forming a layer around the core. The timing of the positioning of the core is crucial for forming a layer having uniform thickness. The equipment used for this positioning is costly, because the core must be centered in the material in its gelatinous state, and at least one of the mold sections, after having material positioned therein, must be turned over and positioned onto its corresponding mold section. As a result casting processes often lead to air pockets and voids in the layer being formed, resulting in a high incidence of rejected golf balls. The cost of rejected balls, complex equipment, and the exacting nature of the process combine to make casting a costly process in relation to injection molding. In addition, the nature of current casting processes is such that materials that require a relatively long time (in comparison to other fabrication methods) to sufficiently solidify, i.e., react thoroughly. As a result, materials or compounds with particular chemistries that react or solidify relatively quickly are generally restricted from use in commercial casting processes, particularly in the golf art.
This has led to the development of alternative fabrication techniques for thermosets such as reaction injection molding (“RIM”). As opposed to traditional injection molding, thermosetting materials and/or materials with relatively quick reaction or solidification times can be processed into certain articles using RIM. For example, U.S. Pat. No. 4,762,322 discloses golf clubs with heads that can be made from a hollow metal shell or a low density, high strength material, such as a reaction injection molded polyurethane, formed around weighted inserts. With respect to manufacture of golf balls, RIM has been disclosed, for example, in International Publication No. WO 00/57962, which claims golf balls, and processes for making such balls, comprising a reaction injection molded material, such as polyurethanes/polyureas. In addition, U.S. Pat. No. 6,083,119 discloses a multi-layer golf ball with an inner and outer cover layer, at least one of which can contain a reaction injection molded polyurethane material.
As mentioned above, by “reactive” it is meant that the polymer is formed from two or more components which react. In reaction injection molding, the two or more components are combined and reacted to produce the final polymerized material. Mixing of these separate components is critical, a distinct difference from traditional injection molding. The process of reaction injection molding a golf ball cover involves placing a golf ball core into a die, closing the die, injecting the reactive components into a mixing chamber where they combine, and transferring the combined material into the die. The mixing begins the polymerization reaction which is typically completed upon cooling of the cover material.
Generally, the components, prior to reacting, exhibit relatively low viscosities. The low viscosities of the components allow the use of lower temperatures and pressures than those utilized in traditional injection molding. However, the rapid subsequent reaction of the two components requires complex mixing heads to be used as precise control of mixing times and reagent stoichiometries.
Finally, in addition to materials which fall into the discrete categories of thermoplastic or thermoset, it has also been found possible to modify a thermoplastic polyurethane or polyurea compositions by introducing materials in the composition which are able to undergo subsequent curing in additional processing steps such as compression molding after injection molding the thermoplastic and result in properties similar to those of a thermoset. For example, Kim et al. in U.S. Pat. No. 6,939,924, the entire contents of which are hereby incorporated by reference, disclose a thermoplastic urethane or urea composition formed as a reaction product of diol or a polyol with an isocyanate; and further comprising a modified or blocked isocyanate, the resulting mixture may then be injection molded to form half shells which can then be placed around a golf ball core and compression molded under conditions sufficient to cause the blocked isocyanate to unblock and induce further cross linking to form a thermoset cover.
However, it would be extremely advantageous to develop a system in which a thermoset composition may be directly formed as a golf ball layer by utilizing a conventional injection molding process and equipment without the need for installation of the complex mixing and control systems typically associated with reaction injection molding and without the need for additional process steps to complete the cross linking process.
The present invention provides an isocyanate-modified composition comprising A) a solid polyol, solid polyamine or a solid polymer containing both hydroxyl- and amine-functionality and B) a blocked isocyanate. The compositions of the present invention may then be injection molded using conventional injection molding techniques at a temperature sufficient to cause unblocking of the isocyanate component and subsequent reaction to form either a thermoplastic or thermoset urethane or urea-modified article, depending upon the stoichiometry of the blocked isocyanate and polyol or polyamine components in the composition, and the time or temperature of the injection molding process.
In an especially preferred embodiment, using the composition of the present invention allows a golf ball layer to be manufactured having the mechanical properties of a thermoset polyurethane but using conventional injection molding techniques and without the requirement for either a single processing step, such as reaction injection molding (with its complex mixing and control systems), or the need for an initial injection molding step followed by a subsequent processing step, (such as compression molding) to form a thermoset.